A population of microtubules grows and shortens asynchronously;
at any instant in time, most are growing and a few are shortening.
The structures of growing and shortening microtubule ends are different:
growing ends have extensions of protofilaments
shortening ends have curling protofilaments that bend back away from the microtubule lattice
7.5 Microtubule assembly and disassembly proceed by a unique process termed dynamic instability